For several years now, it has been known to convert both color and black and white images scanned using a variety of image sensors to an output ink recording of various types of printers, such as ink jet printers. One example of such an image conversion system is disclosed in U.S. Pat. No. 4,672,432 assigned to Canon of Japan, and this system is capable of converting red, blue and green colors of a picture taken by a scanner into cyan, yellow and magenta color outputs from an ink jet printer. The subject matter of this U.S. patent is incorporated herein by reference.
More recently, significant developments have been made in the art of thermal ink jet color printing. An example of one such development was the introduction by the present assignee, Hewlett-Packard Company, in 1987 of its multicolor thermal ink jet printer sold under the trademark "PaintJet". This printer is described in detail in the Hewlett-Packard Journal, Vol. 39, No. 4 published in August 1988 and incorporated herein by reference. Other newly available color ink jet printers have been recently described in numerous trade journals, and an example of such publication is an article in Personal Computing magazine by W. L. Rosch entitled "New Printers Banish Black and White", May 31, 1988, at page 168 et seq.
While these color ink jet printers have been generally well received and operate satisfactorily in most respects, some of the images recorded by these printers have exhibited a certain undesirable color contrast or "grainyness" which detracts from the quality of the recorded image. There are two ways of reducing this "grainyness" or undesirable color contrast, and one way involves using a higher dots per inch (dpi) resolution whereas the other way involves the use of gray scale techniques. The present invention falls into the latter category.